1. Field of the Invention
The invention relates to a piezoelectric thin film element using a piezoelectric layer formed of lithium potassium sodium niobate etc.
2. Related Art
A piezoelectric material is processed into various piezoelectric elements in accordance with different purposes, particularly, it is widely used for functional electronic components such as an actuator for generating deformation by applying voltage or a sensor for generating voltage from the deformation of elements in a reverse way, etc.
As a piezoelectric material used for an actuator or a sensor, a lead-based dielectric material having large piezoelectric characteristics, especially, Pb(Zr1-xTix)O3-based perovskite-type ferroelectric called PZT, has been widely used thus far, and the piezoelectric material is formed by sintering oxide which is generally formed of individual elements.
In addition, in recent years, it is desired to develop a piezoelectric material not containing lead from environmental consideration, and thus, lithium potassium sodium niobate (general formula: (NaxKyLiz)NbO3 (0<x<1, 0<y<1, 0<z<1, x+y+z=1), etc., has been developed. Since the lithium potassium sodium niobate has piezoelectric characteristics comparable to PZT, it is expected as a potential candidate for a lead-free piezoelectric material.
On the other hand, currently, downsizing and high performance are strongly demanded also in the piezoelectric element as downsizing and high performance in various electronic components progress. However, in a piezoelectric element produced by a manufacturing method mainly by a sintering process which is a conventional manufacturing method, especially when a thickness thereof becomes 10 μm or less, a dimension thereof comes close to that of a crystal grain composing a material, and influence cannot be ignored. Therefore, there arises a problem that variation or deterioration in characteristics becomes evident, and a method for forming a piezoelectric material by employing a thin-film technique etc., as an alternative to the sintering process is being researched in recent years in order to avoid such a problem.
Recently, a PZT thin film formed by a RF sputtering method has been put to practical use as an actuator for a high definition inkjet printer head or a compact and low cost gyro sensor (e.g., see JP-A 10-286953 and non-patent literary document of Kiyoshi Nakamura “High performance of piezoelectric material and advanced applied technologies” (in 2007) published by Science & Technology Co., Ltd). In addition, it has been proposed a piezoelectric thin film element using a lithium potassium sodium niobate not using lead (e.g., see JP-A 2007-19302).
It is possible to manufacture a inkjet printer head or a gyro sensor with small environmental burden by forming a lead-free piezoelectric thin film as a piezoelectric thin film. A basic research in thinning of lithium potassium sodium niobate is progressing as a concrete candidate for the lead-free piezoelectric thin film. However, it is difficult to reproducibly manufacture a piezoelectric thin film of lithium potassium sodium niobate having a large piezoelectric constant equivalent to that of PZT.
In addition, in order to reduce the cost in applications, it is essential to establish a technique to controllably form a lead-free piezoelectric thin film on a Si substrate or a glass substrate. When an actuator or a sensor is manufactured using a Si substrate or a glass substrate, it is required to provide electrodes on and under the piezoelectric thin film, however, since surface roughness of top/bottom electrodes and the piezoelectric thin film are rough and surface irregularities are large in the prior art, it is difficult to stably process into a predetermined shape or a surface state when a device is formed by applying a process, thus, there is a problem that a decrease in manufacturing yield occurs. In addition, since the irregularities on device surfaces are large, deterioration in aging characteristics occurs due to generation of leakage current or electric field intensity of a grain boundary portion of a piezoelectric material, and as a result, a decrease in piezoelectric constant or product lifetime occurs.